Has anybody been there, and if so any recommendations or tips?

I'm going on business but the business side of it is about 3-4 days, and seeing as it's the other side of the world and I've never been there before, I'll stay for 2 weeks to have a look around.

At the moment I know I definitely want to go to Phi Phi Islands, but open to other suggestions, willing to venture anywhere really but flights are very long as it is so don't want to waste more time travelling somewhere very far when/if there is nice things to do reasonably close.

Kuala Lumpur is for 3 nights by the way so the bulk of the trip will be in Thailand.